The present invention relates to an improved mould, with improved contact features, for the continuous casting of steel slabs having a thickness in the range of 50-120 mm, particularly suitable to be rolled to thickness values of thin strip, i.e. even less than 1 mm.
German Patent No. 887990 discloses a water-cooled mould for the continuous casting of metallic slabs which in the inlet upper zone is basically in the shape of a funnel with central enlargement, whereinto the submerged nozzle opens, gradually decreasing downwards along the mould, to reach, well before the actual outlet, width values equal to the thickness of the slab leaving the mould.
The successive European Patent No. 0149734 aimed at avoiding a solidification localized in the zone near to the narrow faces, wherein the larger sides converge, to occur as a result of the mould narrowing towards the smaller sides, being funnel-shaped with angularly disposed walls, leading also to the consequence (however not supported by practical experimentation) of the cast flow being blocked. This problem was solved by providing that, at the side of the funnel-shaped casting zone, the larger side walls extend flat and parallel to each other. However, this kind of mould is very likely to involve turbulence problems in the zones with parallel walls, lateral with respect to the central concavity, lacking the desirable draining of refluxes caused by upward directed streams of molten metal from the submerged nozzle. The consequences of this fact are negative for the final product surface quality and affect particularly ultra-thin rolled products because of the powders being trapped in the steel. From DE-A-4031691 a mould for thin slabs is known, having a central hollow or concavity of the two opposite forming plates, which plates show a first section, starting from the inlet zone of the mould, being basically vertical until about half-height, having then a curved profile at the end zone of outlet of the mould, with radius of curvature for the internal or intrados plate which is equal to the one for the external or extrados plate, reduced by the thickness of the thin slab. A mould with plates shaped according to these features was found not to solve the problem of a possible detachment of the casting product from the walls in the sections with a sudden curvature change, although it offers certain advantages with respect to previous moulds, especially as far as cooling homogeneity is concerned.
This brings about a longitudinal discontinuity that not only implies non uniform cooling, but also can cause both compressive and tensile local mechanical stresses, respectively at the intrados and at the extrados, with possibility of cracks or fractures of the skin in the mostly stressed zones, until causing to the so-called xe2x80x9cbreak-outsxe2x80x9d. In order to avoid these troubles Italian Patent No 1265065 in the same applicant""s name modified the longitudinal profile of the mould so that a vertical section of the two forming plates is composed of a certain number of curve lines, connected to each other, having upwards increasing radiuses of curvature to an almost infinite value, with vertical tangent at the inlet.
Unsolved problems of turbulence at meniscus were further tackled in the patent application MI 96A002336 in the same applicant""s name, which provided optimized parameters, at high casting rate conditions, in the form of ratios of the area included between submerged nozzle and mould large faces to the remaining area portions of the same cross-section, as well as between submerged nozzle and smaller sides, and respective parameters defining said areas, trying this way to improve the behaviour at the meniscus without modifying the plates profile in horizontal cross-section.
Other moulds for continuous casting are known for example from EP-A-0658387 and DE-C-4403045, the first one with large faces in the shape of arcs of circle, being convex in cross-section, and the second one at constant concavity, but neither of them having an optimal contact with the slab skin. The same can be said about Japanese published patent application No 51-112730 that provides a mould with large opposed faces having a curved, respectively concave or convex profile, symmetrical with respect to two orthogonal median axes and connected at its ends to a rectilinear profile.
Also EP-A-0611619 discloses a mould for continuous casting with a central cavity having a convex-concave shape, wherein the ratio between the convex radius to the concave radius should be between 1.5 and 3.0. The cavity depth is decreasing towards the mould outlet, but the radius of the central cavity does not increase constantly towards the mould outlet, being constant for a part of the terminal portion. This lack of continuous variation of the radius and the fact that the lateral sections of both large faces are parallel (therefore are not bent) give rise to some discontinuity in guiding the slab skin while maintaining the contact with the mould plates.
This invention aims therefore at providing a mould allowing continuous contact with the slab skin in every point of horizontal and vertical cross-sections, during the withdrawal of the slab. A homogeneous cooling is thus obtained, allowing both a uniform thickness of the skin along the whole profile of the same cross-section and a continuous variation of the thickness according to the height of the varying cross-section, to be achieved, these conditions being ideal to avoid shrinkages and irregular stresses unavoidably leading to longitudinal cracks on the slab surface.
Further, it is desirable to obtain at meniscus level a reduced rate of the upward directed streams of steel at the mould sides to have in these areas very low stationary waves, with remarkable advantages for the surface quality of final products.
This is achieved by a particular concave shape of the mould that gives to its large faces a definite conicity through concave-convex wide bends (therefore, not merely concave or convex like according to the above-mentioned Japanese publication) connecting narrow faces to the central rectilinearly profiled zone of the concavity.